Noiiieuclatwe of the Foraminifera. 391 



exposure of the early whorls, and thus constituting a passage- 

 form between Tr. gordialis and JV. incerta in one direction, 

 as Tr. pusilla is a link in another. 



We may here remark that Trochammma squamata {typica) 

 has a very near relationship to Valvulina in structure and 

 habit, though it possesses more chambers and wants a definite 

 tongue-like appendage at the orifice. This alliance has been 

 suggested to us by our friend and colleague, Mr. H. B. Brady, 

 F.L.S., whose collection of these Foraminifera particularly 

 exemplifies their many intermediate gradations of form. So 

 also Tr. injlata sometimes seems to become Lituola cana- 

 riensis by the increased coarseness of its shell and its more 

 compactly nautiloid shape. Indeed there is no real specific, 

 much less generic, distinction between all these and many 

 other associated forms, if such distinctions fade away as gra- 

 dations of intermediate styles of structure and shape become 

 more and more known. 



Again, though the Miliolce have for the most part a homo- 

 geneous calcareous shell, yet many become coated with a 

 sandy envelope, and, except in the possession of a tongue or 

 valve at the aperture, may be lineal descendants and repre- 

 sentatives of such forms as are here figured in Plate XIII. 

 figs. 9-14 ; whilst Cornuspira and Sjnroloculina, in particular, 

 may in like manner be descended from such as figs. 1-3. 



Whether or not the tongue-like process in Miliola and the 

 valve in Valvulina are essential distinctions, there is no doubt 

 that there is a considerable range of variation in the shell- 

 structure produced by these and other simple Protozoans, 

 and that it is difficult to distinguish the limits between coarse- 

 ness and fineness, roughness and smoothness, when the amount 

 of sand in the shells of some forms ( Valvulina^ Miliola^ Buli- 

 mina^ Textularia, &c.) varies from much to nothing. 



§ X. Trochammina jmsilla is very widely and very plenti- 

 fully distributed in the Permian rocks of England and Ger- 

 many. In Durham it ranges from the lowest beds to the 

 middle of the Magnesian Limestone. It is absent in the 

 highest beds. In Yorkshire it only occurs in the lower beds 

 of the series. 



It usually occurs as casts ; sometimes (in hard subcrystal- 

 nlie limestone) it is seen as sections showing internal struc- 

 ture, and occasionally as well-preserved testiferous specimens. 



In Durham it is found in the " Shell-limestone " at Tunstall 

 Hill, Humbleton Hill, and Claxheugh, near Sunderland. 



In the " Lower Limestone " of the same county it occurs at 

 Hartley's Quarry and Pallion near Sunderland, Westoe, Offer- 

 ton, Rough Dene, Eldon, Langton, Morton Tinmouth, Sum- 



